Metallic biomaterials have been widely used as load-bearing prostheses, internal fixation devices, vascular/nonvascular stents and dental brackets, wires and implants because of their excellent mechanical strength and resilience. Metallic biomaterials are advancing rapidly with the innovative design and development of titanium (Ti)-based biomaterials due to their superior mechanical behavior, chemical properties, and biocompatibility. Even though Ti and its alloys are generally trusted for biomedical applications, there is still room for improvement because of the mechanical mismatch between bone and implant and the need for higher bioactivity. In recent years, much attention has been given to the design of new Ti alloys with improved properties tailoring processes such as heat treatment and severe plastic deformation, surface functionalization for fabricating implants with lower elastic moduli in combination with higher strength and better biocompatibility. Porous Ti has also attracted increasing interest because the 3D porous network can provide a favorable environment for bone in-growth and possess bone-like mechanical properties. Thus far, considerable efforts have been made to develop new manufacturing techniques for producing porous Ti, such as powder metallurgy and the additive manufacturing techniques.

The main objective of this Special Issue is to provide a forum for discussing state-of-the art and novel ideas for producing added-value titanium implants by focusing on new alloys, fabrication, surface treatment and coating of Ti-based biomaterials. This Special Issue presents a thoughtful collection of recent advances and emerging developments in designing and developing biomaterials for hard tissue replacement alternatives. The Special Issue contains 17 papers which cover five areas, including process improvements (3 papers), novel structures or alloy/composite materials (6 papers), new coatings and surface structure modification (7 papers), and localized drug delivery (1 paper). The summary of overview is shown in Fig. 1.

Figure 1
figure 1

Pictorial representation of special issue

Full size image

Process improvements

The paper entitled “Microstructural Characterization and Mechanical Properties of SLM printed Ti-6Al-4V Alloy: Effect of Build Orientation” investigates the effect of selective laser melted (SLM) build orientation (vertical vs. horizontal) on the microstructure, tensile and wear properties of Ti–6Al–4V alloy and compare them with those of wrought alloy [1].In the paper entitled “Remediation of Machining Medium Effect on Biocompatibility of Titanium Based Dental Implants by Chemical Mechanical Nano-structuring”, the biocompatibility of the Ti dental implants machined in oil is compared with that machined in deionized water [2]. The chemical mechanical nano-structuring is also studied, and the fibroblast viability and bacteria growth are assessed. The paper entitled “Influence of cleaning process on mechanical properties and surface characteristics of selective laser melted Ti6Al4V parts prepared for medical implant applications” presents a post-processing cleaning approach for Ti6Al4V parts fabricated by SLM technique, in which glass bead-blasting and vibratory polishing are employed first and a routine cleaning process followed it. The post-treatment surface roughness, surface chemistry, and fatigue life were evaluated [3].

Novel structures or alloy/composite materials

The paper entitled “Nanostructured Ti-13Nb-13Zr for Dental Implant Applications produced by Severe Plastic Deformation” develops a continuous nano-crystalline structure in Ti alloy by employing thermomechanical processing followed by subsequent multi-step heat treatments [4]. In the study entitled “Two Novel Titanium Alloys for Medical Applications: Thermo-mechanical Treatment, Mechanical Properties, and Fracture Analysis” also dedicated thermo-mechanical treatments and suggested two variants of non-toxic medium- to high-strength titanium alloys (Ti–0.44O–0.5Fe–0.08C0.4Si–0.1Au and Ti–0.44O–0.5Fe–0.08C–2.0Mo) [5]. The outcomes reveal these new alloys show excellent mechanical properties and might be a possible alternative for toxic Ti–6Al–4V for use in medical applications. The paper entitled “Ti6Al7Nb-TiB nano-composites for ortho-implant applications” produces Ti–6Al–7Nb-based TiB reinforced composites using a SLM process [6]. The composite with 3.0 wt% TiB is found to have enhanced wear resistance of up to ~ 48% and good adherence and proliferation of cells. In addition, the results demonstrate that Ti–6Al–7Nb composites are promising candidates for the fabrication of biomedical implants and can be easily fabricated by solidification-based additive manufacturing processes. In the study entitled “Fabrication of Ti-Al2O3-HA Composites by Spark Plasma Sintering and its Properties for Medical Applications” the mechanical and biological responses of Titanium–Alumina–Hydroxyapatite (Ti–Al2O3–HA) composites are fabricated by the spark plasma sintering method for dental applications [7]. The results show the addition of alumina and hydroxyapatite to titanium enhances the corrosion resistance and biocompatibility; however, the compressive strength is about 699 MPa. The paper entitled “Biocompatibility and corrosion resistance of low-cost Ti-14Mn-Zr alloys” addresses developing new low-cost Ti-alloys implants with highly biocompatible characteristics [8]. The Mn and Zr alloying elements (Ti–14Mn–xZr alloys) are selected as low-cost alloying elements to investigate their impact on the phase structure, mechanical properties, corrosion resistance, and cytocompatibility. Ti–14Mn–6Zr concerts ultra-high-strength values reached 1830 MPa and high corrosion resistance and cytocompatibility representing potentially low-cost alloys can be applicable in biomedical applications after further development. In the study entitled “Effect of low modulus titanium plate fixation on rabbit femur bone healing” the in vivo response of Ti–6Al–4V ELI (Ti64) and Ti–29Nb–13Ta–4.6Zr (TNTZ) is compared [9]. The result shows that the TNTZ implant plate develops less callus formation than Ti64 plates in the overall bone cross-sections. Thus, the TNTZ implant plate promotes new bone formation with good properties in the early stage of fracture healing.

New coatings and surface roughness modification

The paper entitled “Facile formation with HA/Sr–GO-based composite coatings via green hydrothermal treatment on β-type TiNbTaZr alloys: Morphological and electrochemical insights” investigates the significance of graphene oxide (GO) in the formation of a hybrid composite coating composed of strontium-doped hydroxyapatite (HA) and GO on the surface of β-type TiNbTaZr (TNTZ) alloy via a green hydrothermal method [10]. The results show that at 4.5 wt% of GO, dense homogenous nanocrystalline structure rods with good corrosion-resistant properties are observed. Thus, it is a potential candidate for orthopedic applications. In the study entitled “Sol-gel derived hydroxyapatite coating on titanium implants: Optimization of sol-gel process and engineering the interface” monophasic and crystalline defect-free hydroxyapatite is produced on the substrate of Ti6Al4V [11]. The study shows that introducing TiO2 interlayers eliminate the morphological defects, improve the adhesive strength of HA layers to 76.2%, and exhibits high corrosion resistance. The study entitled “In vitro Assessment of Plasma-Sprayed Reinforced Hydroxyapatite Coatings Deposited on Ti6Al4V Alloy for Bio-implant Applications” presents the in vitro assessment of Ti6Al4V alloy, hydroxyapatite (HA), and calcium silicate (CS) reinforced HA coatings using atmospheric plasma spraying process [12]. The findings of this study indicate that surface modification of Ti6Al4V alloy with hybrid reinforced (HA-10%CS and HA-20%CS) coatings exhibits high microhardness and crack-free morphology, thus promising approach to improve performance for bio-implant applications. The paper entitled “Deposition of Magnesium on Surface Modified Titanium for Biomedical Applications” attempts the surface modification of Ti alloy via four different methods of (a) immersion, (b) single-step anodization, (c) reverse polarization, and (d) reverse polarization followed by anodic oxidation to analyze the feasibility of magnesium (Mg) deposition on the substrate of Titanium [13]. The results show the promising deposition of Mg ions on Ti substrate via reverse polarization followed by the anodic oxidation process. The paper entitled “The Effect of Using Al2O3 and TiO2 in Sandblasting of Titanium Dental Implants” evaluates the effect of Al2O3 and TiO2 abrasive on mechanical (roughness, microhardness, residual stresses and fatigue) and in vivo biological response (osseointegration) [14]. The as-received with acid etching and sandblasted with TiO2 and with Al2O3 with posterior acid etching implants are studied. In the paper entitled “Optimal surface roughness of Ti6Al4V alloy for the adhesion of cells with osteogenic potential”, the influence of surface roughness of cold-wrought Ti–6Al–4V alloy on the adhesion of osteoblastic cells is studied by fluorescence microscopy [15]. The paper entitled “Effect of strontium-doped coating prepared by microarc oxidation and hydrothermal treatment on apatite induction ability of Ti13Nb13Zr alloy in vitro” prepares composite coatings with the different contents of Sr substituted for Ca in hydroxyapatite on the Ti–13Nb–13Zr surface using a combination of micro-arc oxidation and hydrothermal treatment [16]. Hydrophilicity, and apatite formation are studied in detail.

Localized drug delivery

The review paper entitled “Drug-Eluting Titanium Implants for Localised Drug Delivery” thoroughly reviews the progress and research of using Titanium Implants for Drug-Eluting as a new technique for Localized Drug Delivery [17]. Surface modification of implants through coating or adsorption to form porous surface structures as a drug-loaded carrier by polymers, ceramics, or composite are also systematically discussed. The recent trends of controlled drug release and mechanical and physical stability of coated or adsorbed materials are also reviewed.